Entry of the human immunodeficiency virus into target cells involves initial binding of virus to the cellular receptor, the CD4 glycoprotein, and subsequent fusion of viral and cellular membranes and release of the viral core into the cytoplasm. Both binding and fusion steps are mediated by the viral envelope glycoprotein, which in HIV consists of two non-covalently-associated subunits, gp120 and gp41. The high affinity binding of virus to receptor is mediated by gp120, whereas fusion is thought to involve a hydrophobic "fusion peptide" at the amino terminus of the transmembrane subunit, gp41. Details of the interaction between gp120 and CD4 have received considerable attention, since agents that block this binding prevent initiation of the viral life cycle. Little is known, however, of post-binding events that result in membrane fusion. There is mounting evidence that the binding and fusion steps in HIV entry can be uncoupled. There are numerous examples of CD4+ cells (murine cells, for example) that are resistant to infection even though the virus can bind to receptor. This phenomenon appears to involve a block before membrane fusion. Specific sequences within a region of gp120 (the V3 domain) not involved in binding to CD4 have been shown to determine cellular tropism and initiation of the fusion step. These findings suggest that cellular factors other than CD4 need to be matched to specific sequences int he envelope glycoprotein for fusion to proceed. Recent results from several laboratories indicate that the defect in fusion can be complemented in trans in heterokaryons. This indicates that resistance to infection is due to the absence of one or more cellular factors, other than CD4, involved in HIV entry. The proposed experiments will seek to identify novel host cell factors involved in HIV entry using both genetic and immunological approaches. Somatic cell hybrids will be used to identify human chromosomes encoding genes required for viral entry; mutant CD4+ cell lines that have lost the capacity to be infected with HIV will be identified and characterized; and gene transfer approaches will be developed to isolate gene encoding the relevant host factors required for HIV entry. In an alternative approach, screening assays will be developed to identify new monoclonal antibodies that block HIV entry by interfering with cellular factors other than CD4. The results of these studies should provide insight into the complex mechanism of HIV entry and may suggest novel approaches for interfering with the earliest step in the HIV life cycle.